Author Topic: VCA for elements with different valence electrons  (Read 43 times)

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Offline gherasi

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I understood from a previous post that VCA for elements with different valence electrons is not available.
What would you suggest it can be used to estimate the lattice changes for alloys of elements with different # of valence electrons?

Offline Petr Khomyakov

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Re: VCA for elements with different valence electrons
« Reply #1 on: Yesterday at 11:48 »
If your alloy structure is random alloy, you may then use a supercell approach for alloy modeling, combined with special quasirandom structure (SQS) method to avoid doing multiple large-supercell calculations, see Section (and references therein) on QuantumATK application for SiGe alloy in our recently-published QuantumATK reference paper at https://iopscience.iop.org/article/10.1088/1361-648X/ab4007 - it is an open access paper, so you may download right away. Note that SQS methodology is implemented in QuantumATK. You just have to decide on your alloy supercell size, generate an SQS structure of that alloy, and proceed to regular geometry optimization (volume and/or ion relaxation) to get an equilibrium lattice parameters of an alloy for a given alloy composition.

We still have a plugin in the Builder that can create a supercell structure with randomly-placed alloy atoms, e.g., Ge in Si, but then you would need to generate several of those for a given alloy composition to have some statistics. Using SQS allows reducing the computational burden by a smart choice of an alloy structure, but one still have to consider a supercell size large enough to get an accurate description of physical quantities of interest. Volume would probably be not that sensitive to that. So, one may consider a supercell of a moderate size. But that may depend on your system of study.

Offline gherasi

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Re: VCA for elements with different valence electrons
« Reply #2 on: Yesterday at 13:10 »
Is there a tutorial regarding the supercell approach for alloy modeling, combined with special quasirandom structure (SQS) method ?

Offline Petr Khomyakov

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Re: VCA for elements with different valence electrons
« Reply #3 on: Yesterday at 13:52 »
Here is a manual page on SQS, https://docs.quantumatk.com/manual/Types/EvolutionarySQS/EvolutionarySQS.html, see also a reference therein. Otherwise, there is no particular difference between regular supercell calculations and supercell calculations for an SQS structure, as soon as the latter is generated as described in the manual and literature.

I note that there exists GUI support for SQS generation, see the corresponding plugin in the Builder.

Offline gherasi

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Re: VCA for elements with different valence electrons
« Reply #4 on: Yesterday at 22:31 »
Questions:
1) What is the name of the plugin in the Builder that is used for SQS generation?
2) To what version of QuantumATK was the plugin added?
3) Is it possible to add the plugin to an earlier version?
4) Is there an example for how a hexagonal ternary alloy, A(x)B(1-x)C, lattice should be defined before the optimization with EvolutionarySQS ?

Thank you
« Last Edit: Today at 10:49 by gherasi »

Offline Petr Khomyakov

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Re: VCA for elements with different valence electrons
« Reply #5 on: Today at 12:10 »
It is in Builders -> SQS Alloy Builder. It is not possible to add it to older versions, as it is not an Add-On plugin. In fact, it requires the actual calculation using Job Manager, meaning that it is a very special plugin. So if you do not have it, the only way of adding it is to upgrade to a latest version of QuantumATK.

I have attached a png-file with an image how to set up SQS optimization in the Builder. Essentially, you have to create a supercell structure (any symmetry, cubic, hexagonal ..., or with no symmetry at all) of a pristine material in a usual manner, e.g., Si, then select a region which you want to alloy, e.g., all atoms to alloy the entire structure, and choose an alloy element, e.g., Ge, and set composition and other settings, if needed. Send the thing to Job Manager (or the Editor if you want to see the actual script). The SQS configuration will eventually appear as an object on the LabFloor.
« Last Edit: Today at 12:13 by Petr Khomyakov »

Offline gherasi

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Re: VCA for elements with different valence electrons
« Reply #6 on: Today at 12:42 »
How do I get the details related to an upgrade?
It seems that currently the SQS alloy can be defined only for binaries. It this right?
What about ternaries?
« Last Edit: Today at 12:47 by gherasi »

Offline Petr Khomyakov

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Re: VCA for elements with different valence electrons
« Reply #7 on: Today at 13:28 »
- You may apply for a free trial first at https://www.synopsys.com/silicon/quantumatk.html. Which version do you have now?

- You can still do A_x B_1-x C, just select only the elements A and B for mixing in the SQS alloy plugin. It is only if you want to do A_x B_y C_1-x-y, i.e., mixing all the 3 elements at the same time, then there is a limitation of the current implementation.

Offline gherasi

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Re: VCA for elements with different valence electrons
« Reply #8 on: Today at 14:14 »
2016.3 Is the free trial fully functional?
How does one specifies the 3rd element, C?
« Last Edit: Today at 14:44 by gherasi »